This invention relates to a fluoroolefin-alkene base copolymer which is soluble in various organic, solvents, curable at relatively low temperatures and useful as a paint vehicle.
Fluororesins are generally excellent in weather resistance, heat resistance and chemical resistance. Accordingly paints using a fluororesin as the vehicle have been developed for applications to chemical plants, vehicles, buildings and other constructions, machinery and food containers. Recently much attention has been directed to solvent-thinned paints using a fluororesin which is soluble in organic solvents and curable at relatively low temperatures.
It is well known that copolymerization of a fluoroolefin such as chlorotrifluoroethylene or tetrafluoroetylene with an alkene such as ethylene, propylene or butylene gives an alternating copolymer which is high in crystallinity and transparency, relatively high in the content of fluorine and excellent in heat resistance, chemical resistance and weather resistance. For example, British Pat. No. 949,422 and U.S. Pat. No. 2,753,328 show such copolymers.
To use a fluoroolefin-alkene copolymer as the vehicle of a recoatable paint it is necessary to use at least one other comonomer for rendering the copolymer soluble in organic solvents and for introducing a functional group into the copolymer.
JP-A 59-219372 proposes to copolymerize a hydroxyalkylvinyl ether with a fluoroolefin and an alkene to obtain copolymers which have hydroxyl group and hence can be cured with a known curing agent such as a polyisocyanate or a modified melamine resin. However, when a hydroxyalkylvinyl ether is used there is some problem about compatibility of the copolymer with popular curing agents. Moreover, in the case of using a hydroxyalkylvinyl ether it is not easy to prepare a desired copolymer by radical polymerization, and in the presence of proton the alkylvinyl ether is liable to hydrolize into acetaldehyde and alcohol. It is often that during the copolymerization reaction system the reaction liquid containing a copolymerized product gels as the pH of the reaction liquid lowers. To prevent lowering of the pH it is necessary to perform the copolymerization in the presence of a considerable amount of an acid acceptor, but the existence of even a very small amount of the acid acceptor in the product causes clouding of the copolymer solution and insufficient gloss of a coating film formed by application of the copolymer solution.
Often it is desired to introduce a polar group such as carboxyl group, besides hydroxyl group for curing, into a fluoroolefin base copolymer for use as a paint vehicle for the purpose of improving dispersibilities of pigments in solutions of the copolymer. However, in the case of using a hydroxyalkylvinyl ether it is difficult to simultaneously copolymerize the monomers including a carboxyl-containing monomer to a sufficient degree of polymerization and at good yield since the alkylvinyl ether is liable to decompose or react with the carboxyl-containing monomer. It is necessary to first prepare a copolymer not having carboxyl group and then reacting the copolymer with a carboxyl-containing monomer. For example, JP-A 58-136605 proposes first preparing a copolymer of a fluoroolefin and a hydroxyalkylvinyl ether and then reacting the copolymer with succinic anhydride. From a practical point of view, such a two-stage process is inconvenient and raises some problems including a tendency to coloring of the finally obtained copolymer.